This invention relates to the polymerization of water miscible monomers in aqueous media.
Most water-soluble polymers, particularly polymers of acrylamide, acrylic acid and their water miscible derivatives, have been employed commercially as additives in the manufacture of paper products, as water purification coaggulants, as flocculants for a wide variety of aqueous dispersions such as sewages and industrial waste water, as the fluid mobility control agents in enhanced oil recovery, and as treating agents in a wide variety of other applications.
Of the various methods employed to polymerize water miscible monomers, aqueous phase polymerization methods wherein the monomer is dissolved in the aqueous phase at the onset of polymerization are the most commonly employed. Such methods are less expensive and readily provide water-soluble polymers having the high molecular weight that is essential for many applications. In such methods, the concentrations of monomer and resulting polymer in the aqueous phase are maintained as high as possible in order to maximize the capacity of the polymerization apparatus and to minimize the amount of water in the resulting polymer product. Unfortunately, however, polymerization of such water miscible monomers to high molecular weight, or polymerization in solutions of more than 15 weight percent of the monomer often yields gel-like products which are difficult to dilute and which contain a substantial portion of water-insoluble solid. The presence of the latter is particularly troublesome when a polymer is to be employed in enhanced oil recovery applications wherein an aqueous solution of the polymer is passed through a porous subterranean zone in the manner described by U.S. Pat. No. 3,039,529.
As is well known, such aqueous phase polymerizations are normally initiated by a free radical generating catalyst which is activated by the removal of the oxygen that is normally present in the polymerization medium. Of particular interest are the redox catalyst systems which employ an oxidizing free radical generator such as a peroxide or a persulfate and a reducing component such as sulfur dioxide or a bisulfite. While such redox catalyst systems bring about a rapid polymerization, the resulting polymers often have wide molecular weight distribution including substantial quantities of water-soluble gel-like product. In addition, it is often observed that without the addition of high temperature polishing catalysts, as much as 1 or 2 percent of a water-soluble monomer is not polymerized at all and thus must be removed. Finally, and most importantly, the reducing component of the conventional redox catalyst system is often extremely sensitive to oxygen at higher temperatures; therefore, oxygen must be carefully removed prior to the initiation of polymerization. Accordingly, initiation of polymerization is often an unpredictable operation wherein initiation may be rapid and easy to achieve on one occasion and slow and very difficult to achieve on a second occasion under essentially the same conditions. As a result, commercial operation of the polymerization to produce a uniform product is often very difficult.
In view of the problems existing in the prior art of polymerizing water-soluble monomers in an aqueous phase to produce water-soluble polymers, it is highly desirable to provide such a process wherein initiation can be rapidly and uniformly achieved and polymerization can be conducted at a rapid rate to form water-soluble polymer having the desired molecular weight and molecular weight distribution.